This invention relates to a stripping assembly. In particular, it relates to a stripper for removal of a tubular article, such as a can body, from a close-fitting mandrel on which the article is carried, for example for forming.
Stripping devices are in general use for stripping thin-walled cans from a punch or ram on which such cans have been formed in a “draw and wall-ironing” (“DWI”) process. These strippers typically comprise an annulus of stripping fingers. The annulus is situated within a die assembly such that radially inward ends of the fingers extend into the bore of the assembly through which the punch passes during the DWI process.
The stripper fingers are biased inwardly so that on the forward, can forming, stroke of the punch, the can formed and carried on the punch deflects the stripping fingers against the biasing force as the can moves along the bore through the fingers. After the can has moved axially beyond the fingers, the biasing force causes the fingers to be deflected against the punch itself. As the punch moves on the return stroke, the fingers prevent the can from moving with the punch and the can is stripped from the punch.
The biasing force for deflecting the fingers has been provided by a variety of spring devices, most conventionally by the use of an ‘O’ ring of resilient material, although other biasing devices, for example a continuous helical spring or using hydraulics, are also possible.
A known stripping assembly which uses an O-ring for biasing respective stripping fingers is described in GB-B-2,181,685. The O-ring in this patent encircles the stripping fingers and is compressively trapped between the fingers and an outer opposing part of an annular housing. The O-ring further causes the fingers to rest on seating parts of the housing so as to limit the extent to which the tips of the fingers, which engage the can, protrude from the housing.
Known stripping devices have several drawbacks, particularly when high manufacturing speeds are used. If excessive pressure is used to strip the can during the return stroke, the open end of the can may be spoiled and the punch may even be damaged. During the forward stroke, the coating on the can surface is often scratched as the stripper fingers are forced open by the can carried on the punch. Although the stripping assembly of GB-B-2,181,685 is said to apply only a light pressure to the surface of the can and/or punch, it has nonetheless been found that small scratches are made on the can surface as the stripper fingers are forced open by the can carried on the punch on its forward stroke.
“Active” stripping has been proposed in U.S. Pat. No. 5,115,662 in which one or more electromagnets are energized to move the stripping fingers into or away from the stripping location. This device requires the stripping fingers to be formed from magnetically conductive material and is unsuitable for use with cans which are themselves of magnetically conductive material such as steel since any debris arising from the forming process would be attracted to the electromagnet.
The abstract of JP 2003 103312 shows a stripping device which uses a cam actuation to move the stripping fingers away from the can surface on the forward stroke of the punch. Not only does this device require externally moving parts in the form of the cam and cam followers but it also requires external energizing to move the fingers into the stripping position on the return stroke of the punch.
Although stripping using air pressure alone has been proposed, such devices have not proved practical at high speeds and a mechanical stripper acting on the edge of the can is still required. At high line speeds, the stripper may be used in conjunction with an air strip in which pressurized air passes along the center of the punch to assist in can removal and avoid formation of a vacuum which could cause can collapse.